Please don’t forget to tell us what Audacity version you are using (all three numbers, see the pink panel at the top of the page).
If you are using Audacity 2.1.0 there is a bug that you have to install FFmpeg to import MP4 files - see Audacity 2.1.0: FFmpeg required for MP4/M4A/MOV/ALAC import
However if you upgrade to the current Audacity 2.1.2 from Audacity ® | Download for Mac OS you won’t need that workaround. In that version, use the File Menu in Audacity to import the file, or drag the file to the Audacity icon in the Dock. You can’t drag it directly into Audacity. This will be fixed in the next 2.1.3 release.
Gale